Known compact tandem pumps of axial piston configuration, with swash plates and coaxially arranged shafts are designed with the two ends of the pump housing being similar or identical to the front housing of a standard single pump. Between the two end portions of the housings a single or a double mid-housing is inserted in which an auxiliary pump can also be mounted.
The shafts of both pumps and their corresponding cylinder blocks are supported by independent bearings. Accordingly, a bearing is arranged in each of the two end housings and two bearings are arranged in the mid-housing. Each shaft typically has a splined end in the mid housing area and both splined ends are in engagement with a separate splined hollow coupling member to permit torque transmission between the two pumps.
An example of this type of arrangement is disclosed in DE 198 28 180. In that reference the shafts 4 and 5 of the first and second hydraulic pumps 2, 3 are engaged by an additional coupling member 28, and the shafts are supported by four bearings 8, 9, 11, 12.
A similar arrangement is disclosed in JP 9177668. As shown in FIG. 1 of that document, male and female portions of the first and second shafts 24A, 24B form a direct intermeshing connection in the mid housing, such that a separate coupling member is not required. The shafts 24A, 24B are supported by two bearings 38, 39 in two housing end radial covers, and two further bearings 31, 34 in the mid housing region.
One disadvantage of these arrangements relates to the large number of parts required for radial bearing of the shafts and for effecting torque transmission between them. This results in increased manufacturing costs and the overall axial length of the tandem pump assembly is large.
The excessive length of the pump assembly adds to the overall weight of the pump unit, and hence requires additional material during production. These factors contribute to increased manufacturing and operating costs of the pump unit.
Another known arrangement of compact tandem axial pumps utilises two cylinder blocks on a same shaft, which is radially mounted on two bearings. For example, this type of arrangement is shown in DE 42 25 380. In this prior art document, as can be seen in FIG. 1 for example, a primary pump 1 and a secondary pump 12 are both mounted on a single input shaft 2. However, such arrangements are suitable only for lower operating pressures and small displacements of the pumps.
It is an object of the present invention to provide a tandem axial piston pump unit which has a reduced axial length, and/or reduced weight and/or is of a simplified design to conventional tandem axial pump units.